<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            height: 100px;
            width: 100px;
        }
        #div1{
            background: red;
            float: left;
        }
        #div2{
            background: green;
            clear: both;
        }
    </style>
</head>

<body>
   <div id="div1"></div>
   <div id="div2"></div>
</body>

</html>